LENHAM POTTERY MODELS
making high-fired semi-porcelain models since 1969
Additional information on pottery mould making and slip casting ceramic models

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The nature of clay and the use of deflocculants.

Fine sand is 0.2mm dia. ground flint will pass 0.02mm mesh

china clay is flat platelets about 0.002 mm. dia.
You can make fine sea sand (above, on the left) into sand-castles on the beach, because the sand is wet. If the castle dries out, it will blow away because there is too much air between the grains. The grains on the right are like the sand found in glacial deposits, crushed and known as sharp sand.

Commercially ground flint, quartz or feldspar is added to plastic clay to modify it in certain ways. Ground by machinery, it is an extremely fine powder but it is crushed and not rounded by the action of the waves. Fired clay known as 'grog' is also used in casting slip.

The definitive authority on this subject is Daniel Rhodes, with his Clay and Glazes for the Potter, (ISBN -0-8019-5633-1) explains the need for non-plastic material in the slip.

True china clay has been formed by the chemical decomposition of granite rock and this chemical action results in microscopic flat platelets. It is found in this country in Cornwall in its original state, undisturbed by water or the ice of glaciers. It fires white and is known as primary clay. Secondary clay has been picked up by rivers or glaciers, and deposited elsewhere, often stained with iron and contaminated with sand and silt. It is the usual red earthenware found all over England from Cumbria to Kent. Devon Ball Clay has been given a second movement which makes it more plastic than China Clay but it still fires nearly white.

 
Clay with 50% water is slurry. The tiny flat platelets of clay will 'flock' together when water is added to plastic clay. At the proportion of 50% water and 50% clay (by weight) these 'flocs' will settle by gravity and being heavier than water, will sink. Keeping them in suspension with less water is what 'deflocculants' do. Clay with deflocculant is only 30% water. Two soluble alkalies, Sodium Silicate (in syrup form) and Soda Ash (a dry powder) are dissolved in water and added to the bucket when mixing the clay and water. Their action is to reduce the amount of water needed to make the clay into a fluid slip. The deflocculant acts as a kind of lubricant, so that the clay platelets slide over one another.
Plaster of Paris was produced in France by 1743. It was used to make moulds mainly for press-moulding. It was another hundred years before the secret of what we know as casting slip was discovered. Slurry was the usual method of casting liquid clay into plaster moulds until the late nineteenth century, when the science of chemistry found the answer to the problem of too much water. Donal E. Frith in his book, Mold Making for Ceramics, (ISBN 0-8019-7359-7), has chapters on slip-making for American users. Most ceramic supply companies in this country can supply recipes for their ingedients. Non-plastic material such as quartz, feldspar or grog is needed in casting slips to 'open' the mix and allow the water to flow through the tightly packed platelets of plastic clay and be absorbed by the dry plaster mould.

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